Elevator systems may be designed to operate over a specific input voltage range from a power source. The components of the drive of the elevator have voltage and current ratings that may allow the drive to continuously operate while the power supply remains within the designed input voltage range. However, in some scenarios, the local power supply of the utility network is less reliable, such as scenarios in which the utility voltage sags, brownout conditions occur (e.g., voltage conditions below the tolerance band of the elevator drive) and/or power loss conditions become prevalent. When such utility failures occur, the drive draws more current from the power supply to maintain uniform power to the hoist motor. In conventional systems, when excess current is drawn from the power supply, the drive may shut down to avoid damaging components of the drive.
When power sag or power loss occurs, the elevator may become stalled between floors in the elevator hoistway until the power supply returns to the nominal operating voltage range. In conventional systems, passengers in the elevator would be captive until a maintenance worker is able to release a brake for controlling cab movement upwardly or downwardly to allow the elevator to move to the closest floor. Elevator system designs may combat these issues during power sag or loss by employing automatic rescue operations including electrical storage devices that are controlled after power failure to provide power to move the elevator to the next floor for passenger release.
In recent elevator designs, the drive of the elevator may employ a regenerative drive system. A regenerative drive delivers power to the motor from a main power supply during the normal operating condition and delivers power from a backup power supply in the case of a power failure operating condition (e.g., power sag, power loss, etc.). Regenerative drives may include a converter on the input or power utility grid side and an inverter on the motor side, wherein power demand of the inverter is matched by an appropriate power capability on the converter. Such regenerative drives may need strict regulation by a controller to provide available power to the motor and to the backup power supply. Examples of such devices are further detailed in U.S. Patent Publication No. 2012/0261217 (“Regenerative Drive with Backup Power Supply”).
A regenerative drive for an elevator has positive and negative power demands, which means that when the drive has a positive demand it may draw external power (e.g., from a local power source) and when it has negative power demands it produces electricity as a generator. Therefore, the voltages across various components must be strictly regulated and managed in regeneration scenarios, which is when the motor produces energy as a generator in negative power scenarios. A direct current (DC) link may be present, bridging the inverter and converter to smooth power output and buffer the output current of the inverter and converter.
Some recent regenerative drives may operate using a three-phase power input source. In some regenerative drives, the inverter and/or converter may be designed with multilevel topologies (e.g., a three-level regenerative drive topology). In such designs, management of acoustic noise, efficiency, neutral point stability, and thermal balancing is imperative to the success of the design. As such, a need exists to manage the neutral points of the inverter and converter at the DC link.